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Definition: parametric speaker

A loudspeaker that people only hear when directly in front of it. Used for marketing or instructional purposes when someone walks by, parametric speakers use ultrasonic frequencies, which are extremely directional but are beyond human hearing. Whereas a typical loudspeaker pushes air, parametric speakers generate an audible signal by modulating the air outside of the speaker cabinet, which confines itself to the narrow geography of the ultrasonic beam. The higher the ultrasonic frequencies used, the narrower the beam.

Several Ways to Create Audible Sound
By generating two ultrasonic frequencies: one fixed and the other modulated by the audio signal, audible sound is created as they mix. Another way modulates the phase of the second channel. In addition, a single channel of pulse width modulation (PWM) in the ultrasonic range can generate audible sound, and the dual sideband (DSB) method also works by modulating a carrier and then removing the carrier. See PWM, heterodyning and ultrasonic.